1. Field of the Invention
This invention relates generally to a fuel and air injection apparatus and method of operation for use in a gas turbine combustor for power generation and more specifically to a device that reduces the emissions of nitrogen oxide (NOx) and other pollutants by injecting fuel into a combustor in a premix condition.
2. Description of Related Art
In an effort to reduce the amount of pollution emissions from gas-powered turbines, governmental agencies have enacted numerous regulations requiring reductions in the amount of emissions, especially nitrogen oxide (NOx) and carbon monoxide (CO). Lower combustion emissions can be attributed to a more efficient combustion process, with specific regard to fuel injectors and nozzles. Early combustion systems utilized diffusion type nozzles that produce a diffusion flame, which is a nozzle that injects fuel and air separately and mixing occurs by diffusion in the flame zone. Diffusion type nozzles produce high emissions due to the fact that the fuel and air burn stoichiometrically at high temperature. An improvement over diffusion nozzles is the utilization of some form of premixing such that the fuel and air mix prior to combustion to form a homogeneous mixture that burns at a lower temperature than a diffusion type flame and produces lower NOx emissions. Premixing can occur either internal to the fuel nozzle or external thereto, as long as it is upstream of the combustion zone. Some examples of prior art found in combustion systems that utilize some form of premixing are shown in FIGS. 1 and 2.
Referring to FIG. 1, a fuel nozzle 10 of the prior art for injecting fuel and air is shown. This fuel nozzle includes a diffusion pilot tube 11 and a plurality of discrete pegs 12, which are fed fuel from conduit 13. Diffusion pilot tube 11 injects fuel at the nozzle tip directly into the combustion chamber through swirler 14 to form a stable pilot flame. Though this pilot flame is stable, it is extremely fuel rich and upon combustion with compressed air, this pilot flame is high in nitrogen oxide (NOx) emissions.
Another example of prior art fuel nozzle technology is the fuel nozzle 20 shown in FIG. 2, which includes a separate, annular manifold ring 21 and a diffusion pilot tube 22. Fuel flows to the annular manifold ring 21 and diffusion pilot tube 22 from conduit 23. Diffusion pilot tube 22 injects fuel at the nozzle tip directly into the combustion chamber through swirler 24. Annular manifold ring 21 provides an improvement over the fuel nozzle of FIG. 1 by providing an improved fuel injection pattern and mixing via the annular manifold instead of through radial pegs. The fuel nozzle shown in FIG. 2 is described further in U.S. Pat. No. 6,282,904, assigned to the same assignee as the present invention. Though this fuel nozzle attempts to reduce pollutant emissions over the prior art, by providing an annular manifold to improve fuel and air mixing, further improvements are necessary regarding a significant source of emissions, the diffusion pilot tube 22. The present invention seeks to overcome the shortfalls of the fuel nozzles described above by providing a fuel nozzle that is completely premixed, thus eliminating all sources of a diffusion flame, while still being capable of providing a stable pilot flame for a constant combustion process.